In the field of magnetic recording, the desire for higher recording densities is growing in recent years. With respect to coated-type magnetic recording media, various expedients have been proposed for reducing the particle size of ferromagnetic particles, improving the dispersibility thereof, and increasing the loading thereof in a magnetic layer. A more effective expedient presently in use is to employ ferromagnetic metal particles or a hexagonal ferrite which have excellent magnetic characteristics.
On the other hand, magnetic recording disks for use as external memory media have spread remarkably with the spread of minicomputers and personal computers in office appliances, and these magnetic recording disks have come to be used and stored under a wide range of temperature/humidity conditions and also come to be used in a dusty atmosphere. Consequently, it has become necessary to take such various use atmospheres into account in designing such magnetic recording media.
There also is a strong desire for an improvement in recording density in order to attain an increased recording capacity and miniaturization of recording devices. However, in the conventional technique of using acicular magnetic particles to obtain a recording medium suitable for high-density recording, the magnetic particles should be regulated so that the maximum size thereof is sufficiently smaller than the recording wavelength or the length of the recording bit. At present, acicular magnetic particles having a size of about 0.3 .mu.m are already in practical use and, as a result, a minimum recording wavelength of about 1 .mu.m or shorter has become possible.
A further size reduction in acicular magnetic particles is necessary for developing a medium capable of higher-density recording, but such size reduction is difficult because of the following problem. That is, if acicular magnetic particles each having an exceedingly small thickness not larger than 100 .ANG. (angstrom) and hence an exceedingly small volume not larger than 10.sup.-.intg. cm.sup.3 are used in a magnetic coating film, these magnetic particles have reduced magnetic characteristics and are incapable of being sufficiently oriented by application of a magnetic field to the magnetic coating film due to surface properties such as thermal fluctuations.
Although the conventional investigations on ferromagnetic particles for coping with high-density recording have mainly been directed to ferromagnetic metal particles, a magnetic recording medium has been developed which employs as a ferromagnetic material a hexagonal ferrite which is of a platy particle shape and has an axis of easy magnetization in a direction perpendicular to the plate surface (e.g., JP-A-58-6525, JP-A-58-6526). (The term "JP-A" as used herein means an "unexamined published Japanese patent application.") These ferromagnetic particles of a hexagonal ferrite are advantageous in that the above-described problem is mitigated even when the average particle diameter thereof is 0.05 .mu.m or smaller and, hence, the magnetic recording medium employing these ferromagnetic particles is capable of recording at a further heightened density.
On the other hand, investigations have been made on a great reduction in track pitch for a higher recording density. In the field of disk-form magnetic recording media also, use of particles of a ferromagnetic metal or hexagonal ferrite with which a higher output and a higher linear recording density are expected is being investigated in order to meet such desires. Thus, development and practical use of such ferromagnetic particles have been intensively investigated in order to meet the desire for a recording medium having a reduced size and an improved recording density. In particular, there is a desire for thickness reduction and output increase in magnetic layers from the standpoints of attaining a higher recording density and improving the electromagnetic characteristics necessary for overwriting. However, there is a fear that running durability may considerably deteriorate as a result of a thickness reduction.
Illustratively stated, when the thickness of a magnetic layer is reduced in order to improve overwriting properties, the amount of a lubricant which can be infiltrated into the magnetic layer becomes smaller accordingly. As a result, the lubricant is gradually removed to become insufficient due to sliding by a recording/reproduction head, and this may result in, for example, magnetic-layer abrasion, or sticking.
From the standpoint of improving magnetic characteristics, the surface of a magnetic layer should be smoothed more and more. Because of this, the conventionally known lubricants have been ineffective in fully improving running properties, suitability for running repetitions, and durability.
For example, JP-B-54-277723 discloses a technique of using carbonic ester as a lubricant to provide a magnetic recording medium having a long life and high resistance to abrasion and tear. (The term "JP-B" as used herein means an "examined Japanese patent publication.")
Besides the carbonic ester, the conventionally employed lubricants include mineral oils, silicone oils, higher fatty acids, fatty acid esters, animal oils such as beef tallow, whale oil and shark oil, and vegetable oils.
Use of the conventional lubricating oils shown above has had the following problem. If the lubricant is incorporated in a larger amount so as to enhance its lubricating effect, the magnetic coating film comes to have reduced mechanical strength. As a result, the magnetic layer abrades, and the resulting abrasion debris foul the passageway of the recording medium or make it impossible to obtain sufficient still reproduction durability. A known technique for improving still reproduction durability is to use a mixture of fatty acid ester such as butyl stearate and fatty acid such as myristic acid, as disclosed in, e.g., JP-B-28-28367 and JP-B-51-39081. However, the disclosed technique has caused a problem that when the magnetic tape produced using this technique is run under high-humidity conditions, it suffers increased friction and is hence under increased running tension.
Fatty acids, when used alone, are effective in improving image quality but should be used in a large amount in order to obtain lubricity. The incorporation of a large amount of fatty acid has been disadvantageous in that the magnetic layer becomes so soft that the mechanical strength and still reproduction durability thereof are impaired.
The technique of using fatty acid and fatty acid ester compound in combination as described in JP-B-51-39081 brings about satisfactory still durability and relatively reduced tension. However, the recording medium produced with this technique has had a drawback that this recording medium undergoes an increased running tension under high-humidity conditions, e.g., 85% RH (relative humidity).
Although a technique of incorporating an abrasive material (hard particles) into a magnetic layer was proposed as another expedient for improving running durability and has come into practical use, the abrasive material should be incorporated in a considerably large amount because its effect of improving the running durability of the magnetic layer is not easily produced. Namely, it is after all difficult to obtain satisfactory running durability without sacrificing electromagnetic characteristics or head-abrading properties.
In recent years, from the standpoint of obtaining magnetic recording media capable of high-density recording, not only ferromagnetic metal particles but also acicular magnetic particles have come to be incorporated as ferromagnetic particles in magnetic layers having an exceedingly smooth surface. However, such recording media have insufficient running durability.
On the other hand, JP-B-54-27723 discloses carbonic ester (carbonate) having a saturated alkyl group. Since this carbonic ester has a low viscosity for its high molecular weight, fluid lubrication is expected. The proposed carbonic ester also has an advantage of being less susceptible to hydrolysis.
However, the proposed carbonic ester has serious problems when used as a lubricant for coated-type magnetic recording media containing a binder resin. That is, since the carbonic ester is highly compatible with the binder resin, the ester is incapable of behaving in a free state on the magnetic layer and, hence, does not effectively function as a lubricant if merely incorporated in the magnetic layer. In addition, the high compatibility of the ester with the binder resin reduces the strength of the magnetic layer itself, resulting in impaired durability.
As described above, since the amount of a lubricant is limited for attaining a reduction in magnetic-layer thickness, a subject of investigation has been how to produce a lubricating effect.